Shoot Development

A. Function and organization of the apical meristem

B. Origin of primary meristems

C. Gene regulation at the meristem

D. Leaf Initiation

E. Secondary Growth in stems?

Shoot organization - Phytomeres

Phytomer = modular unitof the Shoot

Shoot organization

Protoderm

Apical Meristem

Primary Meristems

Protoderm

Procambium

Ground Meristem

Shoot apical meristem - Importance

• Center of postembryonic growth & development• Source of all primary meristems

– Protoderm, ground meristem & procambium

• Source of – Leaves– Branches– Tendrils– Thorns

• Self-renewing mass of cells stem cells• Balance cell division and cell differentiation

Apical MeristemExamples

Equisetum

Conifers Angiosperms

Shoot apical meristem organization

L1 = tunica

L2 = tunica

Peripheral Zone

Pith or Rib Meristem

Central Zone

Stem Cells

Organizing Center

L3 = corpus

Chimeras & Organization of the Shoot Apical Meristem

Shoot DevelopmentOrganization of the Meristem

1. Periclinal chimeras at the SAM tell us that SAM layers have distinct lineages

1. Nuclear chimeras

2. Genetic markers1. L1 marker: Arabidopsis thaliana meristem layer 1 (ATML1)

2. Mericlinal chimeras at the SAM tell us1. Central zone stem cells are not permanent

2. A few cells (vs many) populate the central zone

3. Domains/tiers of cells may populate the SAM

Copyright ©2004 American Society of Plant Biologists

Laux, T., et al. Plant Cell 2004;16:S190-S202

Gene Expression in the Apical Embryo DomainWUSCHEL (WUS), CLAVATA (CLV) AND SHOOT MERISTEMLESS (STM)

Shoot Development II: GeneticsWUSCHEL gene phenotype

Wild Type

wus mutant

Wild type SAM

wus SAM

Shoot Development II: GeneticsWUSCHEL (WUS) gene Maintains Stem Cell Population

Localization of WUS gene product in

organizing center (OC) of shoot

L1L2L3

Fig 6

wus mutants result in1. early termination of SAM

wus defective in maintaining SAM2. aberrant SAM organization

wus defective in maintaining SAM integrity

WUS function:

1. WUS Protein product is a homeodomain transcription factor

2. Gene regulation

3. Positional influence of once cell type by another

Normal SAM

clavata SAM

Shoot Development II: GeneticsCLAVATA gene phenotype

1. CLV localization in Central Zone of SAM

2. Mutant phenotype: Huge apical meristems CLV wild type restricts stem cell accumulation

CLV localization in Central Zone of SAM

CLAVATA GENE CharacteristicsCLV1 – Extracellular polypeptide: 96 amino acids

Restricted to L1, L2 of SAM Central ZoneCLV2 – Membrane-bound protein receptor with a protein-binding motifCLV3 – Membrane-bound protein receptor with a protein-binding motif and

Restricted to L1, L2 of SAM Central ZoneKinase activity… signaling… Kinase cascadeInhibitory to WUS expression

Shoot Development II: GeneticsCLAVATA gene mechanism

Protein-binding motif

Signal transduction pathway

Gene Interaction: WUSCHEL and CLAVATA Initiation of an Organizing Center in the shoot apical meristem

1. OC precursor lineage established in 4 subepidermal cells of 16 cell proembryo as indicated by expression of WUS (red)

2. Stem cells of Central Zone induced by heart stage as indicated by expression of CLV3 gene (blue)

CLV3 expression

WUS expression

mRNA Expression Domains and gene interaction for CLV1 CLV3 and WUS

clv mutant

Wild type

Wild-type mRNA expression domains illustrate location of gene expression.

1. WUS – under stem cells of Central Zone

2. CLV3 – stem cells of Central Zone above OC (produces extracellular protein)

3. CLV1 – Organizing Center (OC) & vicinity (produces membrane-bound protein)

wus mutant

OverexpressedWUS mutant

1. WUS gene1. Where? Organizing Center of Central Zone (just a few cells)2. Function? Molecular: Encodes homeodomain protein3. Function? Molecular Genetic: Induces Expression of CLV34. Function? Developmental: WUS specifies stem cells of the SAM,

i.e. maintains stem cells and maintains stem cell identity.

2. CLV3 gene:1. Where? Stem cells above Organizing Center 2. Function? Molecular: Encodes peptide secreted in extracellular space3. Function? Molecular Genetic: Inhibits WUS expression.4. Function? Developmental: CLV3 restricts size of Central Zone, i.e. CLV3

restricts size of the stem cell population.

Gene interaction CLAVATA and WUSCHEL in the shoot apical meristem

Gene Regulation: stm mutationSTM wild-type prevents cell

differentiation in Peripheral Zone

STM geneWhere? Stem cells of Central Zone and peripheral Zone.Function? Molecular: Molecular:Encodes homeodomain protein – KNOTTED ClassFunction? Molecular Genetic: Transcription factorFunction? Developmental: Prevents premature differentiation of cells from Peripheral Zone.

stm mutant

Wild type shoot apex

Wild type SAM

stm mutant SAM

Shoot Meristemless (stm) phenotype1. SAM terminates prematurely2. rapid depletion of stem cells, faster than they are replenished

WUS, CLV and STM expression in the shoot apex

1. WUS gene1. Where? Organizing Center of Central Cells (just a few cells)2. Function? Molecular: Encodes homeodomain protein3. Function? Molecular Genetic: Induces Expression of CLV34. Function? Developmental: WUS specifies stem cells of the SAM,

i.e. maintains stem cells and maintains their identity.• CLV3 gene

1. Where? Stem cells of Central Zone2. Function? Molecular: Encodes peptide secreted in extracellular space3. Function? Molecular Genetic: Inhibits WUS expression.4. Function? Developmental: CLV3 restricts size of Central Cells, i.e. CLV3 restricts

size of the stem cell population.1. STM gene

1. Where? Through SAM apical “dome” of cells: central zone and peripheral zone.2. Function? Molecular:Encodes homeodomain protein3. Function? Molecular Genetic: Blocks organ formation genes (AS1, AS2)4. Function? Developmental: Prevents premature differentiation of cells from

Peripheral Zone… thus prevents premature organ initiation.

Summary: Shoot Apical Meristem Genetic Influences on Development

1. Organizing Center (OC): Maintains Stem Cells a. cells expressing WUS gene confers stem cell fate to overlying stem cellsb. cells above OC target stem cells by preferred connections through plasmodesmata

2. Stem Cells control boundaries of stem cellsa. Surgical expts. Demonstrate “release” from inhibition of differentiating “daughter” cellsb. CLV3 gene inhibits WUS gene expression

3. Daughter cells of stem kept in undifferentiated statea. STM gene keeps daughter cells in an undifferentiated state.b. Daughter cells increase to sufficient numbers before organ formation

End Shoot Development